Spacer grid

ABSTRACT

A spacer grid includes interlocked straps comprising metal sheets or plates welded together to define a spacer grid having a top and bottom. The interlocked straps define a plurality of cells comprising vertical passages connecting the top and bottom of the spacer grid. The cells include: upper dimples proximate to the top of the spacer grid and distal from the mid-plane of the spacer grid; lower dimples proximate to the bottom of the spacer grid and distal from the mid-plane of the spacer grid; cantilevered upper springs having fuel rod engagement surfaces proximate to the top of the spacer grid and distal from the mid-plane of the spacer grid; and cantilevered lower springs having fuel rod engagement surfaces proximate to the bottom of the spacer grid and distal from the mid-plane of the spacer grid.

BACKGROUND

The following relates to the nuclear power reactor arts, fuel assemblyconstruction arts, and related arts.

With reference to FIG. 1, a nuclear reactor of the pressurized waterreactor (PWR) variety includes a pressure vessel 10 containing primarycoolant, such as primary coolant water. The illustrative pressure vessel10 is a cylindrical pressure vessel (where “cylindrical” is intended toencompass deviations from a mathematically perfect cylinder such as theillustrative non-uniform diameter of the illustrative cylindricalpressure vessel 10, the inclusion of vessel penetrations, supportstructures, or so forth). A nuclear reactor core 12 is disposed at ornear the bottom of the pressure vessel 10. (Note that in diagrammaticFIG. 1 the pressure vessel 10 is partially cut away as indicated by adashed “opening” in order to reveal the reactor core 12 disposed inside.Moreover, diagrammatic FIG. 1 omits mounting features such as a corebasket that typically are provided to secure the reactor core 12 insidethe pressure vessel 10). Although a PWR is shown in FIG. 1 by way ofillustrative example, it is to be understood that the spacer gridsdisclosed herein are suitably used in nuclear reactors of variousvarieties, such as PWR, boiling water reactor (BWR), and so forth.

The nuclear reactor core 12 typically comprises a plurality of fuelassemblies arranged in a closely-packed array. The fuel assemblyincludes a bundle of vertically oriented fuel rods each comprising afissile material such as ²³⁵U. For example, each fuel rod may containenriched uranium dioxide (UO₂) or mixed UO₂/gadolinium oxide (UO₂—Gd₂O₃)pellets. Interspersed amongst the fuel rods are guide tubes that provideconduits for control rods, instrumentation, or so forth. The top of thefuel assembly is terminated by an upper end fitting or nozzle and thebottom of the fuel assembly is terminated by a lower end fitting ornozzle. The fuel assembly is held together by a plurality of spacergrids including end grids disposed at the top and bottom of the fuelassembly and one or (typically) more mid-grids disposed at spaced apartpositions between the top and bottom of the fuel assembly.

Conventional spacer grids are formed by interlocking orthogonallyoriented metal straps made of sheet metal to define a two-dimensionalgrid of square or rectangular spaces, also called grid “cells”, witheach cell being delineated by four straps. Alternatively, a hexagonalarrangement can be employed in which each cell is generally hexagonaland is delineated by six straps. In one suitable approach employingsquare cells for receiving fuel rods, the strap portions defining eachcell have two dimples formed from the grid straps that form two adjacentwalls of the cell. One dimple in each pair is located near the top ofthe grid strap and the other is located near the bottom of the gridstrap. The opposite walls of the cell each contain a single spring whichmay be formed from the strap that makes that cell wall, or may be aninsert made of a different material that is mechanically trapped orrestrained by features formed from the strap that make up that cellwall. The springs are located at or near the mid-plane of the spacergrid, and are sized such that an interference condition exists when afuel rod is inserted into the grid cell. This interference causes thesprings to deflect backwards towards the cell walls on which they arelocated, preloading the fuel rod in two orthogonal directions againstthe opposing dimple pair and clamping it in position. The axial offsetbetween the plane of action of the springs and the plane of action ofthe dimples creates restoring moments that cause the local verticalorientation of the fuel rod at the spacer grids to remain relativelyfixed should lateral forces be applied to the fuel rod between any twoaxially adjacent spacer grids. In some approaches, each spring contactsits fuel rod at two locations along the length of horizontal or verticalfolds in the spring convolutes. Sometimes local flats, and/or secondaryarches, are also provided to spread out any wear should the fuel rodoscillate in service due to flow-induced vibration. The straps in aconventional spacer grid are typically oriented such that the springs ina given cell are on the outboard walls of the cell and the dimples areon the inboard walls of the cell. This arrangement has the advantage ofproviding a rigid foundation to resist any inward-acting forces that maybe applied to the outer row of fuel rods should the fuel assemblycontact a neighboring fuel assembly or other adjacent structure duringhandling.

BRIEF SUMMARY

In one aspect of the disclosure, a fuel assembly comprises a bundle offuel rods comprising fissile material, and a spacer grid securing thefuel rods of the bundle together. The spacer grid has a top and a bottomand includes a plurality of cells comprising passages from the top tothe bottom of the spacer grid through which fuel rods pass. The cellsinclude: upper dimples disposed proximate to the top of the spacer gridand distal from the mid-plane of the spacer grid; lower dimples disposedproximate to the bottom of the spacer grid and distal from the mid-planeof the spacer grid; upper cantilevered springs having cantilevered freeends located above the mid-plane of the spacer grid; and lowercantilevered springs having cantilevered free ends located below themid-plane of the spacer grid. The upper cantilevered springs of eachcell bias the fuel rod passing through the cell against the upperdimples of the cell and the lower cantilevered springs of each cell biasthe fuel rod passing through the cell against the lower dimples of thecell.

In some fuel assembly embodiments as set forth in the immediatelypreceding paragraph, the spacer grid comprises interlocked strapscomprising metal sheets or plates, and the outermost straps of thespacer grid do not include upper cantilevered springs and do not includelower cantilevered springs. In some fuel assembly embodiments as setforth in the immediately preceding paragraph, the upper and lowerdimples face inward toward the center of the spacer grid and the upperand lower cantilevered springs face outward away from the center of thespacer grid. In some fuel assembly embodiments as set forth in theimmediately preceding paragraph, the upper dimples extend to the top ofthe spacer grid and the lower dimples extend to the bottom of the spacergrid.

In another aspect of the disclosure, a spacer grid includes interlockedstraps comprising metal sheets or plates welded together to define aspacer grid having a top and bottom. The interlocked straps define aplurality of cells comprising vertical passages connecting the top andbottom of the spacer grid. The cells include: upper dimples disposedproximate to the top of the spacer grid and distal from the mid-plane ofthe spacer grid; lower dimples disposed proximate to the bottom of thespacer grid and distal from the mid-plane of the spacer grid;cantilevered upper springs having fuel rod engagement surfaces disposedproximate to the top of the spacer grid and distal from the mid-plane ofthe spacer grid; and cantilevered lower springs having fuel rodengagement surfaces disposed proximate to the bottom of the spacer gridand distal from the mid-plane of the spacer grid.

In some spacer grid embodiments as set forth in the immediatelypreceding paragraph, the cantilevered upper springs have cantileveranchors proximate to the mid-plane of the spacer grid and distal fromthe top of the spacer grid, and the cantilevered lower springs havecantilever anchors proximate to the mid-plane of the spacer grid anddistal from the bottom of the spacer grid. In some spacer gridembodiments as set forth in the immediately preceding paragraph, thecantilevered upper springs have cantilever anchors located at or belowthe mid-plane of the spacer grid, and the cantilevered lower springshave cantilever anchors located at or above the mid-plane of the spacergrid. In some spacer grid embodiments as set forth in the immediatelypreceding paragraph, the fuel rod engagement surfaces of thecantilevered upper and lower springs comprise flat-topped domes. In somespacer grid embodiments as set forth in the immediately precedingparagraph, the outermost straps of the interlocked straps do not includecantilevered upper springs and do not include cantilevered lowersprings.

In some spacer grid embodiments as set forth in the immediatelypreceding paragraph, the set of dimples includes a set of upper dimplesconfigured to contact fuel rods above the mid-plane of the spacer gridand a set of lower dimples configured to contact fuel rods below themid-plane of the spacer grid, and the set of springs includes a set ofupper springs configured to contact fuel rods above the mid-plane of thespacer grid and a set of lower springs configured to contact fuel rodsbelow the mid-plane of the spacer grid.

In another aspect of the disclosure, a spacer grid comprises:interlocked straps of metal sheets or plates welded together to form aspacer grid having a top and bottom, the interlocked straps defining aplurality of cells comprising vertical passages connecting the top andbottom of the spacer grid; and a fuel rods retention system comprising aset of dimples protruding from walls of the cells and a set of springsprotruding from walls of the cells. The set of dimples does not includeany dimples configured to contact fuel rods at the mid-plane of thespacer grid. The set of springs does not include any springs configuredto contact fuel rods at the mid-plane of the spacer grid.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various process operations and arrangements ofprocess operations. The drawings are only for purposes of illustratingpreferred embodiments and are not to be construed as limiting theinvention.

FIG. 1 diagrammatically shows a side sectional view of a pressurizedwater reactor (PWR) according the prior art.

FIG. 2 diagrammatically shows an exploded view of a single fuel assemblyemploying spacer grids as disclosed herein.

FIG. 3 diagrammatically shows a metal sheet or plate undergoingfabrication to form a strap of a spacer grid, wherein cutawaysdelineating springs have been formed in the metal sheet or plate.

FIG. 4 diagrammatically shows a strap formed from the metal sheet orplate of FIG. 3.

FIG. 5 diagrammatically shows Section A-A indicated in FIG. 4.

FIGS. 6 and 7 illustrate a first spacer grid embodiment. FIG. 6diagrammatically shows the arrangement of straps along one direction forthe spacer grid embodiment. FIG. 7 shows a perspective view of thespacer grid embodiment.

FIGS. 8 and 9 illustrate a second spacer grid embodiment. FIG. 8diagrammatically shows the arrangement of straps along one direction forthe spacer grid embodiment. FIG. 9 shows a perspective view of thespacer grid embodiment.

FIGS. 10-12 illustrate a third spacer grid embodiment. FIG. 10 shows aperspective view of the spacer grid embodiment. FIG. 11 shows a planview of an interior strap of the spacer grid embodiment. FIG. 12 shows aplan view of an outermost strap of the spacer grid embodiment.

FIGS. 13-15 illustrate an end-grid embodiment. FIG. 13 shows aperspective view of one corner of the end-grid with the extreme cornercell removed to reveal interior components. FIG. 14 shows a diagrammaticperspective view of one of the horizontally oriented dimples. FIG. 15shows a diagrammatic side view of a pair of upper/lower springs of theend-grid.

FIGS. 16-18 illustrate a mid-grid embodiment. FIG. 16 shows aperspective view of one corner of the mid-grid with the extreme cornercell removed to reveal interior components. FIG. 17 shows a diagrammaticperspective view of one of the vertically oriented dimples disposed onthe outermost straps of the mid-grid. FIG. 18 shows a diagrammatic sideview of a pair of upper/lower springs of the mid-grid.

FIGS. 19 and 20 show two alternative geometries for the upper and lowercantilevered springs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As already set forth, in conventional spacer grids the axial offsetbetween the plane of action of the springs (i.e., soft stops) and theplane of action of the dimples (i.e., hard stops) creates restoringmoments that advantageously cause the local vertical orientation of thefuel rod at the spacer grids to remain relatively fixed should lateralforces be applied to the fuel rod between any two axially adjacentspacer grids. However, it is recognized herein that this same offset ofthe plane of action also induces a bending moment at each spacer gridthat causes the fuel rod to bow in the span between any two axiallyadjacent spacer grids. While this initial bowing may be tolerable, itmay be enhanced by irradiation-induced creep effects during service,potentially resulting in problematic partial or complete closure of theadjacent coolant channels and consequent localized heating.

The conventional orientation of the springs and dimples with the springsin a given cell being on the outboard walls and the dimples on theinboard walls advantageously facilitates resistance to any inward-actingforces that may be applied to the outer row of fuel rods. However, thisarrangement places springs on the outermost strap of the spacer grid,which significantly weakens the outermost straps. This can adverselyimpact the strength of the overall spacer grid structure. It is known toat least partially compensate for this weakness by using a thicker outerstrap compared with the inner straps, and/or via by adding stiffeningribs and other mechanical features to the outermost strap. However,these approaches result in the springs of the outermost strap havingdifferent (typically higher) spring rates as compared with the springsof the interior straps, which enhances the fuel rod bowing effect forthe outermost fuel rods of the fuel assembly, and can lead to excessivein-service fuel rod bow in the outermost row of the fuel rod array.

The centralized location of the springs at or near the mid-plane of thespacer grid is also recognized herein to cause problems. If the folds ofthe springs are oriented vertically, then each spring will actuallycontact the fuel rod at only a single point if there is any misalignmentof the fuel rod due to manufacturing tolerances. This negates theintended benefit of spreading out the contact surface, and can result inhigh local bearing stresses that potentially lead to rapid local wear ifthe fuel rod vibrates in service. Vertically oriented folds also presenta problem during rod loading as they can result in excessive scratchingof the cladding and/or hang-up of the fuel rod and grid damage. Acoining operation is typically applied on the leading and trailing edgesof the spring contacts to ameliorate these problems but it has limitedbenefit.

On the other hand, if the folds are horizontally oriented, the upstreamand downstream spring surfaces typically provide adequate lead-in, socladding scratching and rod hang-up are less of a concern. However, thisorientation provides more local flow blockage, which is less desirablefrom a thermal-hydraulic point of view.

Disclosed herein is a dual cantilever spring arrangement that moves thecontact surfaces between the spring and the fuel rod away from themid-plane of the space grid, and hence toward proximate dimples locatednear the top and bottom, respectively, of the grid strap. The dualcantilever configuration combined with the resulting near-alignment ofthe respective planes of action of the springs (i.e., soft stops) andthe opposing dimples (i.e., hard stops) substantially reduces theinduced fuel rod bow. The cantilevered springs can also be constructedwith low stiffness (i.e., low spring rate). Some disclosed embodimentsalso place the springs on the inboard walls of the cells (with thesprings facing outward), with the dimples located on the outboard walls(and facing inward). This orientation eliminates the spring cutouts onthe outermost strap which substantially strengthens the spacer gridduring lateral impacts.

With reference to FIG. 2, a representative fuel assembly 14 isdiagrammatically shown with partial breakaway and the front top cornerof the perspective view cut away to reveal internal components. The fuelassembly 14 is suitably employed as an element of the nuclear reactorcore 12 disposed in the pressure vessel 10 of FIG. 1. The fuel assembly14 includes an array of vertically oriented fuel rods 20 each comprisinga fissile material such as ²³⁵U. For example, each fuel rod may containenriched uranium dioxide (UO₂) or mixed UO₂/gadolinium oxide (UO₂—Gd₂O₃)pellets. Interspersed amongst the fuel rods 20 are guide tubes 22 thatprovide conduits for control rods, instrumentation, or so forth. The topof the fuel assembly 14 is terminated by an upper end fitting or nozzle24 and the bottom of the fuel assembly 14 is terminated by a lower endfitting or nozzle 26.

The fuel assembly 14 is held together by a plurality of spacer gridsincluding end grids 30 disposed at the top and bottom of the fuelassembly 14 and one or (typically) more mid-grids 32 disposed at spacedapart positions between the top and bottom of the fuel assembly 14.(Said another way, each end spacer grid 30 is closer to an end of thebundle of fuel rods 20 than the mid-grid 32). Illustrative FIG. 2 showsonly two mid-grids 32, but typically additional mid-grids are presentwhich are omitted in the cutaway illustration. The number of mid-grids,and the spacing of the end grids and mid grids along the height of thefuel assembly, is determined based on the total length of the bundle offuel rods, the total number of fuel rods in the bundle, the structuralcharacteristics of the fuel rods, applicable regulatory requirements,and so forth.

With reference to FIGS. 3-7, an illustrative spacer grid isdiagrammatically shown. FIG. 3 shows a metal sheet or plate 40 withcutaways 42 for defining the springs. The metal sheet or plate 40 may,for example, be formed by suitable metal working techniques such asrolling. The metal may be a nickel-chromium alloy (e.g., Inconel), or azirconium alloy (e.g., Zircaloy), or so forth. Inconel is stronger thanZircaloy; however, Zircaloy has a smaller neutron absorptioncross-section as compared with Inconel. Thus, in some embodiments theend grids 30 are made of Inconel while the mid-grids 32 are made ofZircaloy. The cutaways 42 comprise removed portions of the metal sheetor plate, and may be formed by mechanical cutting, laser cutting, or soforth. The cutaways 42 define “templates” for the springs. FIGS. 4 and 5show plan and side sectional views, respectively, of an interior strap44 with: upper dimples 50; lower dimples 52; upper springs 60; and lowersprings 62. These dimples 50, 52 and springs 60, 62 are formed bysuitable deformation of the sheet or plate metal using a press or othersheet metal forming apparatus. FIGS. 6 and 7 show side sectional and topviews, respectively, of the spacer grid comprising interlocked straps44. As seen in the top view of FIG. 7, straps are arranged in twoorthogonal directions to form a two-dimensional grid of cells 70 throughwhich fuel rods (represented by single illustrative fuel rod 20diagrammatically shown in FIG. 7) pass and are retained by the dimples50, 52 and springs 60, 62. (Note that in the top view of FIG. 7, onlythe upper dimples 50, represented by filled spherical caps, and uppersprings 60, represented by open spherical caps, are visible). The strapsare interlocked using mating slots (not shown) that are formed into thestraps at the same time the cutaways 42 are formed. Typically, anassembly jig (not shown) is used to temporarily hold the straps duringassembly of the interlocked structure, and the mating slots are weldedto produce the final stand-alone rigid spacer grid structure.

With particular reference to FIG. 6, the resulting spacer grid has a topat which are disposed the upper features 50, 60, and a bottom at whichare disposed the lower features 52, 62. A mid-plane 66 of the spacergrid is located midway between the top and bottom of the spacer grid.With particular reference to FIG. 5, the springs 60, 62 are formed ascantilevered springs. That is, each upper spring 60 is anchored to thespacer grid at or above the mid-plane 66 of the spacer grid and has acantilevered free end that extends “upward” toward the top of the spacergrid; while each lower spring 62 is anchored to the spacer grid at orbelow the mid-plane 66 of the spacer grid and has a cantilevered freeend that extends “downward” toward the bottom of the spacer grid. Withreference to FIG. 5, this results in an upper “grip plane” containingthe upper features 50, 60, and a lower “grip plane” containing the lowerfeatures 52, 62. (The upper grip plane is not mathematically exactbecause the array of upper dimples 50 is not precisely coplanar with thearray of upper springs 60, and similarly for the lower grip plane). Thesprings 60, 62 do not contact the fuel rods 20 at the mid-plane 66 ofthe spacer grid; rather, the upper springs 60 contact the fuel rodsabove the mid-plane 66 and proximate to the upper dimples 50; andsimilarly the lower springs 62 contact the fuel rods below the mid-plane66 and proximate to the lower dimples 52.

This dual cantilever configuration combined with the resultingnear-alignment of the respective planes of action of the upper springs60 and opposing dimples 50 in the upper grip plane and the resultingnear-alignment of the respective planes of action of the lower springs62 and opposing dimples 52 in the lower grip plane substantially reducesthe induced fuel rod bow. The cantilevered springs 60, 62 can also beconstructed with low stiffness (i.e., low spring rate), with thestiffness being controlled by the dimensions, e.g. width and length, ofthe cantilever (defined by the detailed shape of the cutaways 42) andthe thickness and elasticity of the metal sheet or plate. Since thereare two springs 60, 62, each can have lower stiffness as compared withan arrangement employing a single (higher stiffness) spring.

With particular reference to FIGS. 6 and 7, in addition to the interiorstraps 44, there are two types of “special” straps: four outer straps 44_(O) forming the outer boundaries of the spacer grid; and two mid-straps44 _(M) that intersect at the center of the spacer grid. In theillustrative example of FIGS. 6 and 7, the outer straps 44 _(O) differfrom the interior straps 44 in that they do not include springs 60, 62.As a consequence, the cutaways 42 (see FIG. 3) are not formed into themetal sheets or plates from which the outer straps 44 _(O) arefabricated, and so less metal is removed. This enhances the strength ofthe outer straps 44 _(O), which in turn enhances the strength of theoverall spacer grid structure. In keeping with this configuration forthe outer straps 44 _(O), and as seen in FIGS. 6 and 7, the dimples 50,52 are all arranged on outboard walls of the cells 70, that is, with thedimples facing “inward”, while the opposing springs 60, 62 are allarranged on inboard walls of the cells 70, that is, with the springsfacing “outward”. This design paradigm ideally would place springs onboth opposing sides of the mid-strap. However, as seen in FIGS. 3-5 thesprings 60, 62 are formed by making the cutaways 42 and then deformingthe remaining metal into the cell 70 upon which it is to act. There isonly one upper arm and one lower arm, and so both springs 60, 62 cannotbe formed on opposite sides of the same sheet or plate (that is, onecannot form four such springs in total). In the illustrative mid-straps44 _(M) this is addressed by deforming the upper and lower springs 60,62 in opposite directions, so that the upper spring 60 acts on one cell(e.g., the cell to the left of the mid-strap 44 _(M) in the example ofFIG. 6) while the lower spring 62 acts on the other cell (e.g., the cellto the right of the mid-strap 44 _(M) in the example of FIG. 6).

The mid-strap 44 _(M) of FIGS. 6 and 7 provides only one point ofsupport (either an upper spring 60 or a lower spring 62, but not both).However, the fuel rod remains supported at seven points (four dimplesand three springs). The four center cells would have only six points ofsupport; however, in some embodiments these cells are replaced by acentral guide tube 72 (shown in phantom in FIG. 7). More generally, itis to be appreciated that typically some of the cells 70, or groups ofcells 70, are replaced by guide tubes which are not shown in FIG. 7.Such guide tubes may be used for various purposes, such as providingconduits for control rods, conduits for instrumentation, or so forth.The strap portions surrounding a guide tube optionally do not includesprings or dimples, and omitting these elements around the guide tubesenhances the overall strength of the spacer grid. In some embodimentsstrap portions surrounding a guide tube (that is, a guide tube “cell”)include saddle features (not shown) that position the guide tubeaccurately without generating substantial clamping force. Integral tabson the top and bottom edges of the interior grid straps 44 in thesespecial cells are optionally used to permanently attach the spacer gridto the control rod guide tube during fuel assembly manufacturing. Thisdirect mechanical connection can be made using pulse arc welding,electrical resistance welding, or so forth.

With reference to FIGS. 8 and 9, in an alternative approach twoall-spring (no dimple) interior straps 44 _(M2) arranged in aback-to-back configuration can be used instead of the alternating-springmid-strap 44 _(M). This arrangement provides full eight-point (i.e.,four dimples and four springs) contact for the cells bordered by themid-straps 44 _(M2) (as compared with only seven-point or even six-pointsupport in the embodiment of FIGS. 6 and 7). The back-to-back mid-straps44 _(M2) is also expected to provide some enhancement of the strength ofthe spacer grid. The back-to-back mid-straps 44 _(M2) may have theirbacks in direct contact, or may be slightly spaced apart (as shown inFIGS. 8 and 9).

Another option for the mid-strap, which could replace either themid-strap 44 _(M) of FIGS. 6 and 7 or the back-to-back mid-straps 44_(M2) of FIGS. 8 and 9 is a mid-strap with double dimples (not shown).Both sets of dimples could be formed from a single mid-strap sheet orplate, and would have the same contours as the existing interior strapdimples. This approach avoids the unique strap fabrication die and flowdiversion issues of the double mid-strap 44 _(M2) of FIGS. 8 and 9, andthe partial support issue of the mid-strap 44 _(M) of FIGS. 6 and 7.However, this approach would entail reversing the positioning of thesprings and dimples—that is, the springs would be on the outboard wallsof the cells (with the springs facing inward) and the dimples would beon the inboard walls (and facing outward). This would place springs onthe outermost straps, which is expected to weaken the spacer grid,decrease its impact strength, and make the grid more susceptible tohang-up and tearing during fuel handling.

With reference to FIGS. 10-12, an illustrative embodiment is shown whichincludes structures on the outermost straps 44 _(O) that enhancestructural strength of the spacer grid. Toward this end, the interiorgrid straps 44 include gussets 80 at their outboard ends that interfacewith lead-in tabs 82 disposed on the outer straps 44 _(O). Thesefeatures 80, 82 stabilize the ends of the interior grid straps 44 duringany lateral impact on the fuel assembly such as may occur during seismicevents or shipment. These features 80, 82 also help support the outerstrap lead-in during fuel handling when one fuel assembly is riding upor down along its neighbors in the core. As with the embodiments ofFIGS. 3-9, in the interior straps 44 the strap portions surrounding eachcell contain the dimple features 50, 52 at the top and bottom edges,respectively, sandwiched around the pair of vertically-orientedcantilever spring features 60, 62.

With reference to FIGS. 13-18, some features optionally included on thedimples 50, 52 and springs 60, 62 are illustrated. FIG. 13 shows acorner of an end-grid 30 with the extreme corner cell cut away to revealinterior strap portions defining that removed corner cell. FIG. 14 showsa perspective view of one of the dimples 50, 52, which is horizontallyoriented. FIG. 15 shows an edge profile of the springs 60, 62 of theend-grid 30. Similarly, FIG. 16 shows a corner of a mid-grid 32 with theextreme corner cell cut away to reveal interior strap portions definingthat removed corner cell. FIG. 17 shows a perspective view of a dimple502 _(V) of the outermost strap, which in this case is verticallyoriented. FIG. 18 shows an edge profile of the springs 60, 62 of themid-grid 32.

With particular reference to FIGS. 13-15, the dimples 50, 52 arehorizontally-oriented (see especially FIG. 14). The cantilever springfeatures 60, 62, have a large elastic deflection range due to thecantilevering arrangement. The springs 60, 62 are formed with their mainsurfaces inclined relative to the remainder of the vertical cell wall 84(diagrammatically shown by a vertical dashed line in FIG. 15) so as tocreate a substantial interference with the fuel rod. When the fuel rodis inserted into the spacer grid during manufacturing, these dual springfeatures 60, 62 are elastically deflected back towards the vertical cellwall 84, creating a clamping force that pins the fuel rod against theopposing dimple pair 50, 52. This same clamping action is simultaneouslyactuated at 90° around the cladding by the spring and dimple features inthe perpendicular cell walls.

Because the rod contact surfaces on the springs 60, 62 are at the topand bottom ends of the respective upper and lower springs 60, 62, thefour spring features in the cell bear on the fuel rod cladding inhorizontal planes that are nearly coplanar with the opposing dimplefeatures 50, 52. This mechanical arrangement generates the maximumrestoring moment on the fuel rod for a given spring clamping force(because the moment arm is maximized in the cantilevered arrangement)while minimizing the local bending moment (due to the cooperatingdimple/spring arrangements being co-planar) that can induce rod bowbetween grids.

Integral lead-in surfaces are provided at the top and bottom edges ofthe dimples 50, 52 and springs 60, 62 to facilitate fuel rod insertionboth during manufacturing in the factory and during fuel reconstitutionin the field. The lead-in surfaces on the dimples 50, 52 are formed bycoining the top and bottom edges of the formed dimples 50, 52 at anangle. In the end-grid 30 of FIGS. 13 and 15, the lead-in surfaces onthe springs 60, 62 comprise three-dimensional flat-topped domes 90. Thedome provides a low likelihood of fuel rod hang-up during insertion, andthe flat top of the dome spreads out cladding wear.

In the embodiment of FIGS. 13-18, the end grids 30 (FIGS. 13-15) aresuitably made of Inconel while the mid-grids 32 (FIGS. 16-18) aresuitably made of Zircaloy. Inconel is a more ductile material thanZircaloy, but Zircaloy is preferable for the mid-grids 32 due to itssmaller neutron absorption cross-section as compared with Inconel. TheInconel end grids 30 shown in FIGS. 13-15 employ the flat-topped domes90 as the lead-in/fuel rod engagement surfaces for the springs 60, 62.On the other hand, yield in forming these flat-topped domes 90 in themore brittle Zircaloy material is expected to be low, as the Zircaloysheet or plate tends to tear or develop an orange-peel defect whenformed in three dimensions.

Accordingly, in the Zircaloy mid-grids 32 of FIGS. 16-18, thelead-in/fuel rod engagement surfaces for the springs 60, 62 areconstructed as shallow two-dimensional hooks 92 (see especially FIGS. 16and 18). These hooks 92 are suitably formed by bending the free ends ofthe springs 60, 62 so as to include a flat rod engagement portion and adistal “bent-back” portion that facilitates lead-in. In the case ofeither the flat-topped domes 90 or the hooks 92, these lead-in featureswork in conjunction with a bullet-shaped bottom end plug shape of thefuel rod to reduce the likelihood of hang-up during rod insertion.

In the embodiments of FIGS. 13-18, the outermost straps 44 _(O) of thespacer grids 30, 32 again contain only dimples, but not springs. Thisconfiguration reduces the amount of material that is cut away in formingthe outermost straps 44 _(O). Grid impact behavior is heavily influencedby the strength of the outermost straps, and strong outermost strapsthat are structurally stable are advantageous for achieving high lateralimpact strength. The lead-in features 80, 82 described with reference toFIGS. 10-12 are incorporated in modified form into the embodiments ofFIGS. 13-18. In these latter embodiments the gussets 80 are omitted onthe interior straps 44, and the lead-in tabs 82 disposed on theoutermost straps 44 _(O) engage the corners of the interior straps 44.In all of the embodiments of FIGS. 10-18, these engagements serve astiffening function. They increase the effective moment of inertia ofthe outer strap 44 _(O) while also helping to stabilize the ends of theinterior straps 44.

In the mid-grids 32 of FIGS. 16-18, the dimples on the outermost straps44 _(O) are modified to further enhance the strength of these outermoststraps 44 _(O). Specifically, the horizontally oriented dimples 50, 52(see, e.g. FIGS. 13 and 15) are replaced by vertically oriented dimples502 _(V) in the outermost straps 44 _(O) of the mid-grid 32 of FIGS.16-18 (see especially FIGS. 16 and 17). The vertically oriented dimples502 _(V) are less favorable from the point of view of pressure drop.However, they eliminate four lines of weakness along the length of theoutermost strap 44 _(O) that are present in the case of thehorizontally-oriented dimples 50, 52. The use of the vertically orienteddimples 502 _(V) in the case of the mid-grids 32 is an accommodation tothe lower strength of the Zircaloy construction material. The end-grids30 (FIGS. 13-15) are constructed of stronger Inconel material, andaccordingly the outermost straps 44 _(O) of the end-grids 30 use thehorizontally oriented dimples 50, 52 in the illustrative embodiment ofFIGS. 13-15.

The spacer grids of FIGS. 10-18 can employ any suitable arrangement forthe mid-strap, such as the illustrative “alternating spring” mid-strap44 _(M) of FIGS. 6 and 7, or the back-to-back mid-straps 44 _(M2) ofFIGS. 8 and 9. In the latter case, the back-to-back mid-straps 44 _(M2)occupy additional lateral space, and accordingly the spring lead-infeatures may need to be made shallower for these mid-straps 44 _(M2) inorder to leave sufficient room for insertion of the fuel rods into theproximate cells.

In the disclosed embodiments, once the fuel rod is installed the majorportion of the length of the cantilevered springs 60, 62 is orientedessentially vertically (e.g., parallel with the vertical wall 84 inFIGS. 15 and 18), thereby presenting a limited frontal area to thecoolant flow. The horizontal orientation of the dimples 50, 52 likewisepresents a limited frontal area to the flow. The illustrative spacergrids also do not include flow mixing vanes on the interior grid straps44. Accordingly, the pressure drop across the spacer grid is limited.Spacer grids with reduced pressure drop, such as those disclosed herein,are of particular value in the context of nuclear reactors that employnatural circulation or assisted or forced circulation with a relativelylow flow rate, such as small modular reactor (SMR) designs which aretypically designed to produce 300 megawatts or less of electrical power.

The dual cantilever springs 60, 62 rotate so as to bring the contactsurface (e.g., the flat-topped dome 90 or hook 92 in two illustrativeembodiments) into an approximately line contact with the fuel rodcladding. The illustrative contact surfaces 90, 92 each provide a long“flat” to maximize the contact length and spread out any cladding wearthat might occur at this interface. In the case of the flat-topped dome90, this flat is the flat top of the dome, while in the case of the hook92 a portion of the hook structure is made flat. Likewise, the dimples50, 52 are suitably fabricated with a long flat region between thecoined lead-ins to provide a line contact with the fuel rod cladding tomaximize the contact length and spread out any cladding wear that mightoccur at that interface. The contour of all of the contact surfaces isso formed as to rapidly increase the wear area as metal is removed, soas to compensate for any alignment imperfections that might exist at thecladding-to-grid interfaces. When combined with the two “extra” contactsurfaces that the dual cantilever spring arrangement provides, thedisclosed spacer grids have a large metal volume removed-to-depth ofpenetration ratio, thus providing good contact with the fuel rodswithout unduly compromising overall strength of the spacer grid.

With reference to FIGS. 19 and 20, the dual cantilever design can employdifferent configurations. FIG. 19 shows an embodiment in which thecantilevers are elongated to be side-by-side. Thus, upper springs 60′have their anchors below the mid-plane 66 of the spacer grid and theircantilevered engagement surface above the mid-plane 66; while, lowersprings 62′ have their anchors above the mid-plane 66 of the spacer gridand their cantilevered engagement surface below the mid-plane 66. Theorientation of the two springs 60′, 62′ in any given cell alternatessuch that the root or anchor of the upper spring 60′ is positioned atthe same elevation as the thin hook (or flat-topped dome, or otherengagement surface) on the adjacent lower spring 62′. This allows thedual springs 60′, 62′ to be nested into less space on the strap 44. FIG.20 shows an embodiment in which upper and lower springs 60″, 62″ areoriented horizontally. The orientation of the two springs 60″, 62″ inany given cell alternates such that the root or anchor of the upperspring 60″ is positioned vertically above the hook (or flat-topped domeor other engagement surface) of the lower spring 62″, again enablingnesting into a smaller space on the strap 44. The vertical stacking ofthe springs 60″, 62″ is reversed from cell to cell to balance thetwisting moments applied to the interior grid straps.

The illustrative spacer grids have square or rectangular symmetry in aplane transverse to the fuel rods. However, various disclosed aspectsare readily incorporated into spacer grids of other geometries, such ashexagonal spacer grids having hexagonal symmetry in a plane transverseto the fuel rods.

The preferred embodiments have been illustrated and described.Obviously, modifications and alterations will occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

We claim:
 1. A fuel assembly comprising: a bundle of fuel rodscomprising fissile material; and a spacer grid securing the fuel rods ofthe bundle together, the spacer grid comprising a plurality ofinterlocked straps, having a top and a bottom and including a pluralityof cells comprising passages from the top to the bottom of the spacergrid through which fuel rods pass, the cells including: a plurality ofupper dimples disposed proximate to the top of the spacer grid anddistal from a mid-plane of the spacer grid, a plurality of lower dimplesdisposed proximate to the bottom of the spacer grid and distal from themid-plane of the spacer grid, upper cantilevered springs defined by aplurality of upper cutaways, the upper cantilevered springs beinganchored to the spacer grid by a single base in a plane of acorresponding strap and at the mid-plane of the spacer grid and havingcantilevered free ends located above the mid-plane of the spacer grid,and lower cantilevered springs defined by a plurality of lower cutaways,the lower cantilevered springs being anchored to the spacer grid by asingle base in a plane of a corresponding strap and at the mid-plane ofthe spacer grid and having cantilevered free ends located below themid-plane of the spacer grid, wherein the upper cantilevered springs ofeach cell bias the fuel rod passing through the cell against the upperdimples of the cell and the lower cantilevered springs of each cell biasthe fuel rod passing through the cell against the lower dimples of thecell, and wherein one of the plurality of upper dimples and theplurality of lower dimples is contiguous with the plurality of uppercutaways and the plurality of lower cutaways, respectively.
 2. The fuelassembly of claim 1, wherein the spacer grid is a mid-grid and the fuelassembly further comprises: end spacer grids further securing the fuelrods of the bundle together, each end spacer grid being closer to an endof the bundle of fuel rods than the mid-grid, each end spacer gridhaving a top and a bottom and including a plurality of open cellsthrough which fuel rods pass from the top to the bottom, the cellsincluding: upper dimples disposed proximate to the top of the end spacergrid and distal from the mid-plane of the end spacer grid, lower dimplesdisposed proximate to the bottom of the end spacer grid and distal fromthe mid-plane of the end spacer grid, upper cantilevered springsanchored to the end spacer grid at the mid-plane of the spacer grid andhaving cantilevered free ends located above the mid-plane of the spacergrid, and lower cantilevered springs anchored to the end spacer grid atthe mid-plane of the spacer grid and having cantilevered free endslocated below the mid-plane of the spacer grid.
 3. The fuel assembly ofclaim 2, wherein: the cantilevered free ends of the upper and lowercantilevered springs of the end spacer grids comprise flat-topped domes,and the cantilevered free ends of the upper and lower cantileveredsprings of the mid-grid comprise hooks.
 4. The fuel assembly of claim 3,wherein: end spacer grids comprise interlocked nickel-chromium alloystraps; and the mid-grid comprises interlocked zirconium alloy straps.5. The fuel assembly of claim 1, wherein the upper dimples extend to thetop of the spacer grid and the lower dimples extend to the bottom of thespacer grid.
 6. The fuel assembly of claim 5, wherein the uppercantilevered springs do not include any portion of the mid-plane of thespacer grid, and the lower cantilevered springs do not include anyportion of the mid-plane of the spacer grid.
 7. The fuel assembly ofclaim 1, wherein the spacer grid comprises interlocked straps comprisingmetal sheets or plates, and the outermost straps of the spacer grid donot include upper cantilevered springs and do not include lowercantilevered springs.
 8. The fuel assembly of claim 1, wherein the upperand lower dimples face inward toward the center of the spacer grid andthe upper and lower cantilevered springs face outward away from thecenter of the spacer grid.
 9. The fuel assembly of claim 1, wherein thecantilevered free ends of the upper and lower cantilevered springs arearranged as pairs and the upper and lower dimples are arranged as pairsto collectively define a fuel rod biasing system that is symmetric aboutthe mid-plane of the spacer grid.
 10. The fuel assembly of claim 1,wherein the bundle of fuel rods has one of rectangular, square, andhexagonal symmetry in a plane transverse to the fuel rods.
 11. A fuelassembly comprising: a bundle of fuel rods comprising fissile material;and a spacer grid securing the fuel rods of the bundle together, thespacer grid comprising a plurality of interlocked straps, having a topand a bottom and including a plurality of cells comprising passages fromthe top to the bottom of the spacer grid through which fuel rods pass,the cells including: a plurality of upper dimples disposed proximate tothe top of the spacer grid and distal from a mid-plane of the spacergrid, a plurality of lower dimples disposed proximate to the bottom ofthe spacer grid and distal from the mid-plane of the spacer grid, uppercantilevered springs defined by a plurality of upper cutaways, the uppercantilevered springs being wherein each upper cantilevered spring isanchored to the spacer grid at a single cantilever anchor located in aplane of a corresponding strap and at or above the mid-plane of thespacer grid and extends upward toward the top of the spacer gridterminating in a cantilevered free end located above the mid-plane ofthe spacer grid, and lower cantilevered springs defined by a pluralityof lower cutaways, the lower cantilevered springs being wherein eachlower cantilevered spring is anchored to the spacer grid at a singlecantilever anchor located in a plane of a corresponding strap and at orbelow the mid-plane of the spacer grid and extends downward toward thebottom of the spacer grid terminating in a cantilevered free end locatedbelow the mid-plane of the spacer grid, wherein the upper cantileveredsprings of each cell bias the fuel rod passing through the cell againstthe upper dimples of the cell and the lower cantilevered springs of eachcell bias the fuel rod passing through the cell against the lowerdimples of the cell, and wherein one of the plurality of upper dimplesand the plurality of lower dimples is contiguous with the plurality ofupper cutaways and the plurality of lower cutaways, respectively. 12.The fuel assembly of claim 11 wherein: the upper dimples and the uppercantilevered springs cooperate to define an upper grip plane containingthe upper dimples and the upper cantilevered springs that grips the fuelrods; and the lower dimples and the lower cantilevered springs cooperateto define a lower grip plane containing the lower dimples and the lowercantilevered springs that grips the fuel rods.
 13. The fuel assembly ofclaim 11 wherein the spacer grid comprises interlocked straps comprisingmetal sheets or plates welded together, the passages from the top to thebottom of the spacer grid being defined between the straps.
 14. The fuelassembly of claim 1 wherein the spacer grid comprises interlocked strapscomprising metal sheets or plates welded together, the passages from thetop to the bottom of the spacer grid being defined between the straps.